diff --git a/app/admin/write/page.tsx b/app/admin/write/page.tsx
index 2e2cff6..270d456 100644
--- a/app/admin/write/page.tsx
+++ b/app/admin/write/page.tsx
@@ -9,7 +9,7 @@ export const generateMetadata = async (): Promise<Metadata> => {
 
 const BlogWritePage = () => {
   return (
-    <section className={'p-6 max-w-7xl mx-auto'}>
+    <section className={'py-6 px-2 max-w-8xl mx-auto'}>
       <BlogForm />
     </section>
   );
diff --git a/app/api/posts/[slug]/route.ts b/app/api/posts/[slug]/route.ts
index 83ad40e..aa42ee3 100644
--- a/app/api/posts/[slug]/route.ts
+++ b/app/api/posts/[slug]/route.ts
@@ -47,9 +47,17 @@ export async function PUT(
     const body = await req.json();
     const post = await Post.findOne({ slug: params.slug });
 
+    let thumbnailOfPost = getThumbnailInMarkdown(body.content);
+    if (!thumbnailOfPost) {
+      const seriesThumbnail = await Series.findById(body.seriesId).select('thumbnailImage');
+      thumbnailOfPost =
+        seriesThumbnail?.thumbnailImage ||
+        '/images/placeholder/thumbnail_example2.webp';
+    }
+
     const updatedPost = await Post.findOneAndUpdate(
       { slug: params.slug },
-      { ...body, thumbnailImage: getThumbnailInMarkdown(body.content) },
+      { ...body, thumbnailImage: thumbnailOfPost },
       {
         new: true,
         runValidators: true,
diff --git a/app/entities/post/write/BlogForm.tsx b/app/entities/post/write/BlogForm.tsx
index cdeffda..0f5d0da 100644
--- a/app/entities/post/write/BlogForm.tsx
+++ b/app/entities/post/write/BlogForm.tsx
@@ -240,7 +240,7 @@ const BlogForm = () => {
   };
 
   return (
-    <div className={'px-4'}>
+    <div className={'px-2'}>
       <h1 className={'text-2xl text-center mb-4'}>
         글 {slug ? '수정' : '작성'}
       </h1>
@@ -294,7 +294,11 @@ const BlogForm = () => {
         <MDEditor
           value={formData.content}
           onChange={(value) => setFormData({ content: value })}
-          extraCommands={[calloutCommand, commands.divider, ...commands.getExtraCommands()]}
+          extraCommands={[
+            calloutCommand,
+            commands.divider,
+            ...commands.getExtraCommands(),
+          ]}
           height={500}
           minHeight={500}
           visibleDragbar={false}
@@ -304,8 +308,13 @@ const BlogForm = () => {
               'data-color-mode': theme,
             },
             components: {
-              callout: ({ emoji, children }: { emoji?: string; children?: React.ReactNode }) =>
-                <Callout emoji={emoji}>{children}</Callout>,
+              callout: ({
+                emoji,
+                children,
+              }: {
+                emoji?: string;
+                children?: React.ReactNode;
+              }) => <Callout emoji={emoji}>{children}</Callout>,
             } as any,
             rehypeRewrite: (node, index?, parent?) => {
               asideToCallout(node);
diff --git a/app/hooks/post/usePost.ts b/app/hooks/post/usePost.ts
index 03317b1..07fccc5 100644
--- a/app/hooks/post/usePost.ts
+++ b/app/hooks/post/usePost.ts
@@ -80,7 +80,10 @@ const usePost = (slug = '') => {
     try {
       const data = await getAllSeriesData();
       setSeriesList(data);
-      setFormData((prev) => ({ ...prev, seriesId: data[0]._id }));
+      // 편집 모드(slug 존재)에서는 getPostDetail이 seriesId를 설정하므로 덮어쓰지 않음
+      if (!slug) {
+        setFormData((prev) => ({ ...prev, seriesId: data[0]._id }));
+      }
       setUIState((prev) => ({ ...prev, seriesLoading: false }));
     } catch (e) {
       console.error('시리즈 조회 중 오류 발생', e);